@Aniel Challenges and trial missions are not scaled based on pink stars, so you don't have to worry about that. The only missions that are affected by promotion to pink stars are the scavenge missions.

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To get a survivor to 6 stars (one pink star) directly after promotion it costs 500 tokens each trait upgrade and 1000 tokens to promote. That's 3500 tokens from upgraded 5 star to upgraded 6 star.

You will receive 1% increase on traits like luck, dodge, bullet dodge etc; and a mismal 40-50 atk/health increase upon promotion to which you can then continue to upgrade the survivor but this time it will cost 1000 tokens each trait upgrade for the same nominal 1% increases.

1,000 tokens to promote - and, then 1,000 tokens to upgrade each trait.Although the stats upgrades are kinda crappy, where you notice the improvement is on body shot% at higher levels - the more pink stars you have, the higher level RSL zombies you can attack without body shots.